ABSORPTION-CELLS ON LEAVES. 235 
other Plumbaginez, resemble in an extraordinary degree those pertaining to saxi- 
frages. The depressions are here found uniformly distributed over the entire sur- 
face of a leaf, and when they are closed by a erust or scale composed of calcium 
carbonate, the leaves are dotted with white spots, as may be seen in the drawing 
of a leaf of Acantholimon Senganense given in fig. 55%. Upon the calcareous scale 
being removed, a little cavity is revealed beneath, and one observes that the floor of 
this cavity is composed of from four to eight cells, separated by radial partition- 
walls, and with exceedingly thin and delicate outer walls. The other epidermal 
cells adjoining the cavity are, on the contrary, always furnished with a thick cuticle 
(see fig. 55°). Whenever water is being copiously supplied to the roots, and the 
turgidity of the cells in the leaves is great, the cells forming the floor of the cavity 
excrete bicarbonate of lime in solution. Part of the carbonic acid escapes into the 
air, and the insoluble mono-carbonate of lime in the water then forms a crust, which 
fills and covers the cavity, and often even spreads over the whole leaf, constituting a 
coherent calcareous coat. 
All Plumbaginee which exhibit this contrivance—that is to say, the various 
species of Acantholimon, Goniolimon, and Statice—inhabit steppes and deserts, 
where in summer no rain falls for months together, and the soil becomes dry to a con- 
siderable depth, so that extremely little water is available for the roots. Although 
the rigid leaves are protected by a thick cuticle, and by crusts and scales of lime 
against excessive evaporation of their aqueous contents, still it is difficult to avoid 
some slight loss of water, especially when the noon-day sun beats down upon the 
steppe, and, owing to the extremely arid nature of the soil, it is scarcely possible to 
replace this loss, however small it may be, by absorption from the earth on the part 
of the suction-cells on the roots. All the more welcome to plants of the kind is the 
dew which sometimes falls copiously on steppes and in deserts in the course of the 
night; it wets the rigid leaves, and, soaking immediately underneath the crusts and 
seales of lime to the thin-walled cells at the bottom of the cavities, is absorbed with 
avidity by them. When drought returns with the day, the scales of lime close 
tightly down like lids on the epidermis beneath, and, so far as possible, prevent 
evaporation. In particular, they impede the exhalation of water from the thin- 
walled cells at the bottom of the cavities—a loss which would otherwise be quite 
inevitable, and would be followed by a rapid desiccation of the entire plant. To 
prevent the calcareous lids from dropping off, there are either, as in Saairaga 
Aizoon, papilliform or conical projections from cells in the immediate vicinity of 
the cavities, which projections often have hooked ends and confine the erust of 
lime, or else each cavity is somewhat contracted at the top and enlarged below, so 
that the lime stopper, being shaped according to the contour of the cavity, cannot 
fall out. 
A significance similar to that attributed to calcium carbonate excretions belongs 
also to the saline crusts which are found covering the leaves of a few plants grow- 
ing on the arid ground of steppes and deserts in the neighbourhood of salt lakes 
and on the dry tracts of land near the seashore. Owing to the fact that in these 
